Gas burning consumer and commercial appliances, for example hot water heaters, furnaces, stoves, etc. include various control and safety mechanisms to ensure safe operation thereof. One such safety control circuit used to ensure that the uncombusted release of gaseous fuel does not occur, or if occurring is minimized, is a flame sense circuit. Such circuitry utilizes the rectification property of a flame to detect its presence or absence to control the flow of fuel to the burner of the appliance.
Such flame sensing is used to ensure that the release of gaseous fuel is being combusted at the burner during periods that heating is required. Depending on the control mechanism and programming, the flame sense input may be used simply to determine whether proper combustion is occurring, or may be utilized as a control input to re-trigger the flame ignition circuitry to attempt to relight the flame. In some systems, the absence of flame when heating is commanded will result in a shutdown of the system and possible lockout.
The flame sense circuitry is also utilized to detect the presence of flame when no combustion event is commanded to identify possible failures in the gas control valves. If such a flame is detected when no combustion is commanded, the appliance will typically enter a purge or lockout mode of operation and will signal a failure so that service personnel may be alerted to the potential failure within the system.
Typical flame sense circuits for use in appliances that utilize electronic microprocessor- or microcontroller-based control utilize two separate pins on the microcontroller for each flame sense circuit in a flame rectification detection system. The first pin of the microcontroller is used as an input that reads the charge state of a capacitor that changes whenever a flame is present. The second pin of the microcontroller is used as an output to allow the flame capacitor to recharge to the “no flame” state whenever a flame has been successfully detected.
The controller allows gas to flow to the burner so long as the system can continually verify the presence of flame using these two microcontroller pins. In other words, the microcontroller reads the input pin to determine if flame is present, resets the flame sense circuit with the output pin, reads the input pin to make sure flame is still present, etc. so long as the combustion event is commanded. If at any point during the combustion event, flame is not detected on the input pin after it has been reset by the output pin, the controller knows that a problem has occurred resulting in the flame being extinguished.
In a complete cycle, therefore, the electronic gas controller initially monitors the input pin to verify that no flame is present when the gas has not been commanded to flow. Assuming that this step is successfully passed, the controller energizes the electronic gas control valve and the ignition circuitry to allow the gaseous fuel to flow to the burner and be ignited by the ignition circuitry. This ignition circuitry may be a direct spark ignition (DSI), hot surface ignition (HSI), or other ignition method known in the art. Assuming successful ignition of the gaseous fuel, the flame sense circuit will detect the presence of flame, and the electronic controller will read the input to verify that a flame has been detected. The controller continues to allow gas to flow since it has verified that a flame is present. To ensure that a flame continues to burn during the entire combustion event, the microcontroller resets the flame sense circuit to the no flame state, and then waits a predetermined period of time to verify that the flame sense circuit has again detected the presence of flame. This process continues during the combustion event so long as the microcontroller continues to verify that flame is present each time after the flame sense circuit has been reset.
If, however, the flame sense circuit does not detect the presence of flame after it has been reset, the microcontroller either reinitiates the ignition circuitry to attempt to reignite the gaseous fuel, or commands the electronic gas control valve to turn off to stop the flow of gaseous fuel to the burner, depending on the programming of the system. In any event, if the gaseous fuel is unable to be ignited as determined by a failure of the flame sense circuit to detect the presence of flame, the system will enter a lockout and will typically provide an alert that a failure has occurred so that the appliance may be serviced.
While such flame sense circuits and methodologies work well, the increasing complexity of such appliances driven by the increase in number of features and cycles, as well as the highly cost competitive nature of consumer and commercial appliance industry, have caused designers to critically analyze every aspect of the appliance design to identify potential areas for simplification and cost reduction. Unfortunately, because the detection of flame is such a critical safety feature in consumer and commercial gas burning appliances, continuously being able to reset and re-verify the presence of flame has precluded changes in such circuitry. With some gas burning appliances having multiple burners, e.g. some ranges have two ovens, possibly each with a broiler, and multiple surface burners, the number of pins dedicated to flame sense becomes excessive. Further, increasing demands on utilization of microcontroller real estate, i.e. the utilization of pins on the microcontroller, has caused many manufactures to move to much more expensive, larger microcontrollers in order to add additional features while maintaining the required safety margin in such gas burning appliances.
In view of the above, there is a need in the art for a system and method of reliably detecting the presence of flame and continually being able to verify its continued presence during a combustion mode of operation while reducing the design footprint and complexity, while maintaining the required reliability, of such circuits. The system and method of the present invention provide such a flame sense circuit and method.